Preparation of ultra high molecular weight polyethylene

ABSTRACT

Particulate ultra high molecular weight polyethylene (pUHMWPE) are disclosed having an intrinsic viscosity (IV) of at least 4 dl/g, a molecular weight distribution M w /M n  of less than 4.0, a median particle size D50 of between 50 and 200 μm, a residual Ti-content of less than 10 ppm, a residual Si-content of less than 50 ppm, and a total ash content of less than 1000 ppm.

This application is a divisional of commonly owned U.S. Ser. No.15/029,871, filed Apr. 15, 2016 (now U.S. Pat. No. 9,771,440), which isthe US national phase of International Application No.PCT/EP2014/072837, filed Oct. 24, 2014, which designated the US andclaims priority to EP Patent Application No. 13190212.4, filed Oct. 25,2013, the entire contents of each of which are hereby incorporated byreference.

The invention relates to a method for the preparation of a particulateultra high molecular weight polyethylene (pUHMWPE). The inventionfurther relates to a pUHMWPE obtainable by said process, to a pUHMWPEand to the use of said pUHMWPE.

A process for the preparation of an UHMWPE is disclosed inWO2009/063084. WO2009/063084 discloses a process for the preparation ofultra high molecular weight polyethylene by preparing a single sitecatalyst supported by a magnesium containing carrier, said catalystcomprising an organometallic compound and an activator, and polymerizingethylene in the presence of the single site supported catalyst.

The process disclosed in WO2009/063084 provides UHMWPE polymers withgood mechanical properties and process ability. However, the processesdescribed in WO2009/063084 show poor productivity of the catalystsupported by a magnesium containing carrier. Productivity of single sitesupported catalysts are known to have low productivity especially whencompared to the well established Ziegler-Natta systems. Drawbacks of thelow productivity provided the process as described in WO2009/063084 maybe an economically unviable processes, a particulate UHMWPE with highash content, high residual titanium content and/or small particle sizes.

U.S. Pat. No. 6,528,671 describes a homogeneous catalyst system for theproduction of polyolefins comprising phosphinimine transition metalcompound, a co-catalyst and optionally an activator. However suchhomogeneous catalytic polymerization process is subject to fowlingproblems while the produced polyethylenes are bare of any morphology,rendering their use in commercial processes difficult at best.

EP0890581 describes the production of particulate polyethylene using aphosphinimine cyclopentadienyl metal compound on a silica. The reportedproductivity of the catalyst system is achieved at the detriment of theparticle size and morphology of the polyethylene powder. Thenon-spherical morphology and high particle size of the polyethylenepowder is detrimental for a commercial use in the field of UHMWPE.

Furthermore technology reviews in the field of supported single sitecatalyst, i.e. organometallic catalyst supported by a carrier, have beenpublished in the last decades. Such reviews are for example reported aschapter 4 and chapter 6 of the book “Tailor-made polymers. Viaimmobilization of alpha-olefin polymerization catalysts” by John R.Severn and John C. Chadwick, 2008 WILEY-VCH, Weinheim or Immobilizingsingle-site alpha-olefin polymerization catalysts in Chem. Rev. 2005,105, 4073-4147. These reviews highlight the fact that in the field ofpolyolefin polymerization, no empirical routes apply and each processand product needs a specific a support/catalyst/cocatalyst combinations.

Therefore, there is a need for a process for preparing pUHMWPE havinggood balance between UHMWPE process ability and productivity of theemployed supported organometallic compound.

It is an objective of the present invention to provide a process forpolymerization having a good balance between pUHMWPE process ability andproductivity.

Surprisingly, the objective is reached by the process according to thepresent invention, which will be described in detail below.

In one embodiment of the present invention, there is provided a processfor the preparation of particulate UHMWPE with a MgCl₂ supportedsingle-site titanium catalysts according to claim 1.

Surprisingly with the process according to the invention a particulateUHMWPE with good process ability is obtained at a good productivity ofthe employed supported organometallic compound. The particulate UHMWPEshows a narrow molecular weight distribution combined with low ashcontent and low residual titanium content. A further advantage is thatthe particulate UHMWPE obtained by the process of the present inventionmay have an optimized particle size (D50) of between 50 and 200 μm witha narrow particle size distribution (SPAN). This results in particulateUHMWPE that can be beneficial for further processing such as sintering.

In particular the process for preparation of a particulate ultra highmolecular weight polyethylene of the present invention comprises thesteps of

-   -   a) preparing a magnesium containing carrier by interaction of a        solution of an organomagnesium compound having composition MgR¹        ₂.nMgCl₂.mR² ₂O where n=0.37-0.7, m=1.1-3.5, R¹ is each an        aromatic or aliphatic hydrocarbyl residue and R² ₂O is an        aliphatic ether, with a chlorinating agent at a molar ratio        Mg/Cl of at most 0.5, wherein Mg represents the Mg of the        organomagnesium compound and Cl the Cl of the chlorinating        agent;    -   b) loading the magnesium containing carrier with a        organometallic compound forming a supported catalyst,    -   c) contacting the supported catalyst with at least ethylene        under polymerization conditions,    -   wherein the organometallic compound is a compound of the formula        R³ ₃P═N—TiCpX_(n),    -   wherein    -   each R³ is independently selected from the group consisting of        -   a hydrogen atom,        -   a halogen atom,        -   a C₁₋₂₀ hydrocarbyl radicals optionally substituted by at            least one halogen atom,        -   a C₁₋₈ alkoxy radical,        -   a C₆₋₁₀ aryl or aryloxy radical,        -   an amido radical,        -   a silyl radical of the formula —Si—(R⁴)₃        -   and a germanyl radical of the formula —Ge—(R⁴)₃    -   wherein each R³ is independently selected from the group        consisting of hydrogen, a C₁₋₈ alkyl or alkoxy radical, C₆₋₁₀        aryl or aryloxy radicals,    -   Cp is a cyclopentadienyl ligand;    -   X is an activatable ligand and n is 1 or 2, depending upon the        valence of Ti and the valence of X, preferably, X is selected        from the group consisting of Cl, Br, Me and Et if n=2 or X is a        substituted or unsubstituted butadiene if n=1.

By activatable ligand in the context of the present invention isreferred to a ligand which may be activated by a cocatalyst or“activator” (e.g. an aluminoxane) to facilitate olefin polymerization.Activatable ligands may independently be selected from the groupconsisting of a hydrogen atom, a halogen atom, a C₁₋₁₀ hydrocarbylradical, a C₁₋₁₀ alkoxy radical, a C₅₋₁₀ aryl oxide radical and; each ofwhich said hydrocarbyl, alkoxy, and aryl oxide radicals may beunsubstituted by or further substituted by a halogen atom, a C₁₋₈ alkylradical, silyl radical, a C₁₋₈ alkoxy radical, a C₆₋₁₀ aryl or aryloxyradical, an amido radical which is unsubstituted or substituted by up totwo C₁₋₈ alkyl radicals; a phosphide radical which is unsubstituted orsubstituted by up to two C₁₋₈ alkyl radicals. The preferred catalystscontain a Ti atom in the highest oxidation state (i.e. 4+). Thus, thepreferred catalyst contains two activatable ligands. In some instances,the metal of the catalyst component may not be in the highest oxidationstate. For example, a titanium (III) component would contain only oneactivatable ligand.

Two X ligands may also be joined to one another and form for example asubstituted or unsubstituted diene ligand (i.e. 1,3-diene); or adelocalized heteroatom containing group such as an actetate oracetamidinate group.

In a convenient embodiment of the invention, each X is independentlyselected from the group consisting of a halide atom, an alkyl radicaland a benzyl radical

The preparation of the magnesium containing carrier is preferablycarried out at a temperature in the range from 5 to 80° C. Thepreparation is typically carried out in dry aprotic, hydrocarbonsolvent.

The chlorinating agent can be any type of chlorinated compound able toprovide at least one chlorine to the organomagnesium compound,preferably compounds with covalently bound chlorine such as chlorinatedaliphatic or aromatic hydrocarbons. Preferably the chlorinating agent isa chlorine-containing compound of composition Y_(k)ACl_(4-k), whereY=OR⁵ or R⁵ group with R⁵ being a C₁₋₂₀ hydrocarbyl radicals optionallysubstituted by at least one halogen atom, A being Si or C atom and k=0to 2. Preferably the chlorinating agent is a hydrocarbyl halide silanewith A=Si, k=1 or 2 or phenyl trichloro methane. Magnesium containingcarriers produced by said preferred chlorinating agents have theadvantage of being obtained in higher yield or deliver supportedcatalysts with tunable average particle size, particle sizedistribution, and further increased productivity.

The organomagnesium compound MgR¹ ₂.nMgCl₂.mR² ₂O may have a broadmolecular composition defined by the ranges of n=0.37-0.7 and m=1.1-3.5.It will be obvious to the skilled person that said compounds will havecomplex structures allowing for the decimal nature of n and m.Preferably n=0.4-0.65, more preferably 0.45 to 0.6 and m is preferably1.5-2.5, more preferably 1.8-2.1 and most preferably m=2. Each R¹ is anaromatic or aliphatic hydrocarbyl residue. Preferably R¹ is and arylgroup (Ar), wherein the aryl group preferably comprises an aromaticstructure with 6 to 10 carbon atoms. Optionally the aromatic structureis substituted by at least one C₁ to C₈ alkyl group. Aliphatic R¹residues may be selected from linear or branched alkyl residues with 1to 10 carbon atoms such as methyl, ethyl, n-propyl, i-propoyl, n-butyl,i-butyl or t-butyl. R² ₂O is an aliphatic ether. By aliphatic ether isunderstood that R¹ is a hydrocarbyl residue bound to the oxygen etheratom via an aliphatic carbon atom. Preferably R² is R=i-Am, n-Bu.

In a preferred embodiment, the molar ratio of Mg of the organomagnesiumcompound to Cl of the chlorinating agent in the preparation of themagnesium containing carrier is at most 0.4, more preferably at most0.3. The molar ratio is not particularly limited but for practicalreasons the ratio may be at least 0.01, more preferably at least 0.02and most preferably at least 0.05.

By the process of the invention, a magnesium containing carrier isobtained after step a). Said intermediate magnesium containing carriermay be isolated and stored for later use or subjected immediately to thefurther steps of the process. Preferably, the magnesium containingcarrier has particle size, expressed as D50, from 2 to 30 μm. By D50 isunderstood in the present application the median particle size diameter.Preferably the D50 is at least 2 micron, more preferably at least 3micron and most preferably at least 5 micron. A D50 of the magnesiumcontaining carrier within the preferred ranges may provide an optimumhandling of the obtained particulate UHMWPE in commercial equipment. Themagnesium containing carrier preferably has a narrow particle sizedistribution (SPAN) from 0 to 10, more preferably from 0.1 to 5, evenmore preferably from 0.3 to 2 and most preferably from 0.4 to 1. In afurther preferred embodiment, the particle size distribution is at most5, preferably at most 2 and most preferably at most 1, bearing in mindthat a monomodal particle size distribution has the value of 0.

A supported catalyst is prepared by loading the magnesium containingcarrier with a organometallic compound. This is generally performed bytreatment of a suspension of the magnesium containing carrier with asolution of the organometallic compound in a hydrocarbon solvent. Themolar ratio of the organometallic compound to Mg may be from 0.001 to 1,preferably from 0.005 to 0.5 and most preferably from 0.01-0.1. Theloading of the magnesium containing carrier is preferably carried out ata temperatures between 20° C. and 100° C. Preferably the magnesiumcontaining carrier is suspended in the solvent used for the preparationof said carrier in step a) and may further contain residues from thecarrier preparation step. In a preferred embodiment, reagents from thesynthesis are removed from the carrier by for example washing andfiltration with suitable solvents before the carrier is isolated as apowder or brought again into a suspension. The solvent of the suspensionand the solvent of the organometallic compound may be the same ordifferent. Suitable solvents for the suspension and/or solution of theorganometallic compound are aliphatic and aromatic hydrocarbons,preferably with a boiling point between 20° C. and 200° C. Preferablythe solvents are individually selected from the list consisting ofpentane, hexane, heptane, isoparafine mixture, toluene and xylene aswell as their isomers.

The organometallic compound is of the general formula R³ ₃P═N—TiCpX_(n),wherein each R³ is independently selected from the group consisting of ahydrogen atom,

a halogen atom, a C₁₋₂₀ hydrocarbyl radicals optionally substituted byat least one halogen atom, a C₁₋₈ alkoxy radical, a C₆₋₁₀ aryl oraryloxy radical, an amido radical, a silyl radical of the formula—Si—(R⁴)₃ and a germanyl radical of the formula —Ge—(R⁴)₃ wherein eachR³ is independently selected from the group consisting of hydrogen, aC₁₋₈ alkyl or alkoxy radical, C₆₋₁₀ aryl or aryloxy radicals,

Cp is a cyclopentadienyl ligand; X is an activatable ligand and n is 1or 2, depending upon the valence of Ti and the valence of X, preferably,X is selected from the group consisting of Cl, Br, Me and Et if n=2 or Xis a substituted or unsubstituted butadiene if n=1.

The preferred phosphinimine ligand (R³ ₃P═N—) are those in which each R³is a hydrocarbyl radical. A particularly preferred phosphinimine ligandis tri-(tertiary butyl) phosphinimine (i.e. where each R³ is a tertiarybutyl group).

As used herein, the term cyclopentadienyl ligand is meant to broadlyconvey its conventional meaning, namely a ligand having a five carbonring which is bonded to the metal via eta-5 bonding. Thus, the term“cyclopentadienyl” includes unsubstituted cyclopentadienyl, substitutedcyclopentadienyl, unsubstituted indenyl, substituted indenyl,unsubstituted fluorenyl and substituted fluorenyl. An exemplary list ofsubstituents for a cyclopentadienyl ligand includes the group consistingof C₁₋₁₀ hydrocarbyl radical (which hydrocarbyl substituents areunsubstituted or further substituted); a halogen atom, C₁₋₈ alkoxyradical, a C₆₋₁₀ aryl or aryloxy radical; an amido radical which isunsubstituted or substituted by up to two C₁₋₈ alkyl radicals; aphosphido radical which is unsubstituted or substituted by up to twoC₁₋₈ alkyl radicals; a silyl radical of the formula —Si—(R⁴)₃ and agermanyl radical of the formula —Ge—(R⁴)₃ wherein each R⁴ isindependently selected from the group consisting of hydrogen, a

C₁₋₈ alkyl or alkoxy radical, C₆₋₁₀ aryl or aryloxy radicals. PreferablyCp is unsubstituted cyclopentadienyl or pentamethylcyclopentadienyl.

In a most preferred embodiment of the process of the invention theorganometallic compound is of the formula ^(t)Bu₃P═N—TiCp*X₂, whereinCp* is pentamethylcyclopentadienyl and X is selected from the groupconsisting of Cl, Br, Me and Et. It was found that the supportedcatalyst prepared with such organometallic compound may results inhigher productivity of particulate UHMWPE.

In a further preferred embodiment, the process of the inventioncomprises the step of treating the magnesium containing carrier, theorganometallic compound and/or the supported catalyst with an activatorselected from the list of alumoxanes, alkyl aluminums, alkyl aluminumhalides, anionic compounds of boron or aluminum, boranes such astrialkylboron and triarylboron compounds, borates, and mixtures thereof,preferably the activator is alumoxanes or alkyl aluminums. Well knownboranes and borates used as activator in the preparation of polyolefinsare described in Chem. Rev., 2000, 100, 1391 by E. Y-X. Chen and T. J.Marks which is herein included by reference. By such treatment evenhigher yields of particulate UHMWPE may be obtained.

The alumoxane may be of the overall formula: (R⁵)₂AlO(R⁵AlO)_(p)Al(R⁵)₂wherein each R⁵ is independently selected from the group consisting ofC₁₋₂₀ hydrocarbyl radicals and p is from 0 to 50, preferably R⁵ is aC₁₋₄ radical and p is from 5 to 30. Methylalumoxane (or “MAO”) in whichmost of the R groups in the compounds of the mixture are methyl is thepreferred alumoxane. Alumoxanes are also readily available articles ofcommerce generally as a solution in a hydrocarbon solvent.

The alumoxane, when employed, is preferably added at an aluminum totitanium mole ratio of from 20:1 to 1000:1. Preferred ratios are from50:1 to 250:1.

In the context of the present invention alkyl aluminums and alkylaluminum halides are represented by the general formula AlR⁶ _(q)X_(r),wherein Al is an aluminium atom in the trivalent valence state, each R⁶is an alkyl residue, X is a halogen atom, q and r are natural numbers of1-3 with the provision that q+r=3. Typical examples are trialkylaluminum such as triethyl aluminum, triisobutylaluminum, trihexylaluminum and trioctyl aluminum; dialkyl aluminum hydrides such asdiethyl aluminum hydride and diisobutyl aluminum hydride; dialkylaluminum halides such as diethyl aluminum chloride; mixtures of atrialkyl aluminum and a dialkyl aluminum halide such as a mixture oftriethyl aluminum and diethyl aluminum chloride.

In the context of the application, trialkylboron compounds andtriarylboron compounds are boranes represented by the general formulaBR⁷ ₃, wherein B is a boron atom in the trivalent valence state and eachR⁷ is respectively an alkyl or aryl residue. Preferably the alkyl oraryl residues are substituted with at least on halogen atom. A mostpreferred triaryl boron compound is tris pentafluorophenyl borane.

In the context of the present application borates are boron containingcompounds of the formula [R⁸]⁺[B(R⁹)4]⁻ wherein B is a boron atom, R⁸ isa cyclic C₅₋₇ aromatic cation or a triphenyl methyl cation and each R⁹is independently selected from the group consisting of phenyl radicalswhich are unsubstituted or substituted with from 1 to 5 substituentsselected from the group consisting of a fluorine atom, a C₁₋₄ alkyl oralkoxy radical which is unsubstituted or substituted by fluorine atoms;and a silyl radical of the formula —Si—(R¹⁰)₃; wherein each R¹⁰ isindependently selected from the group consisting of a hydrogen atom anda C₁₋₄ alkyl radical.

In step c) of the process of the present invention the supportedcatalyst is contacted with at least ethylene under polymerizationconditions.

The polymerization may be carried out under various conditions such asat temperatures between 20° C. and 100° C. in the presence of an inerthydrocarbon solvent, such as a C₅₋₁₂ hydrocarbon which may beunsubstituted or substituted by a C₁₋₄ alkyl group such as pentane,methyl pentane, hexane, heptane, toluene, octane, cyclohexane,methylcyclohexane, pentamethylheptane and hydrogenated naphtha or undergas phase conditions without a hydrocarbon diluent at temperatures from20 to 160° C., preferably from 40° C. to 100° C. Low temperatures mayresult in a decreased polymerization activity, whereas high temperaturesmay result in degradation of the catalyst system, lowering of themolecular weight and/or a loss of the particulate morphology of theUHMWPE due to a loss of cohesion of the produced UHMWPE. The polymermolecular weight may be regulated by the use of chain transfer agentsand/or hydrogen. A polymerization pressure is not particularly limited,and a pressure of usually ambient pressure to about 10 MPa, preferablyabout 50 kPa to 5 MPa and most preferably from about 100 kPa and 1.5 MPais adopted, from the industrial and economic viewpoints. As apolymerization type, either a batch type or a continuous type can beemployed. Preferably slurry polymerization using an inert hydrocarbonsolvent such as propane, butane, isobutane, pentane, hexane, heptane andoctane, or gaseous polymerization can be employed. Generally thereactors should be operated under conditions to achieve a thoroughmixing of the reactants. Preferably a batch process is used with theadvantage that particle size distribution as well as, molecular weightdistribution and monomer feeds may be advantageously controlled.

The monomers used in the process according to the invention for thepreparation of the particulate UHMWPE may be dissolved/dispersed in thesolvent either prior to being fed to the reactor or for gaseous monomersthe monomer may be fed to the reactor so that it will dissolve in thereaction mixture. Prior to mixing, the solvent and monomers arepreferably purified to remove potential catalyst poisons such as wateror oxygen. The feedstock purification follows standard practices in theart, e.g. molecular sieves, alumina beds and oxygen and CO(S) removalcatalysts are used for the purification of monomers. The solvent itselfas well is preferably treated in a similar manner.

The feedstock may be heated or cooled prior to feeding to thepolymerization and the reaction may be heated or cooled by externalmeans.

Generally, the catalyst components may be added as a separate suspensionto the reactor or premixed before adding to the reactor.

After the polymerization, polymerization can be stopped by adding apolymerization terminator such as alcohols, water, oxygen, carbonmonoxide or carbon dioxide, removing the monomers, or stopping theaddition of the monomers.

In a yet preferred embodiment of the invention in step c) ethylene andat least one olefinic co-monomer, preferably an alpha-olefin iscontacted with the supported catalyst. In the context of the presentinvention an alpha-olefin refers to an alpha-olefin having 3 or morecarbon atoms, preferably from 3 to 20 carbon atoms. Preferredalpha-olefins include linear mono-olefins such as propylene, butene-1,pentene-1, hexene-1, heptene-1, octene-1 and decene-1; branchedmono-olefins such as 3-methyl butene-1,3-methyl pentene-1 and 4-methylpentene-1; vinyl cyclohexane, and the like. These alpha-olefins may beused alone, or in a combination of two or more. In addition, a smallamount of a compound having multiple unsaturated bonds such asconjugated diene or non-conjugated diene may be added during thecopolymerization.

The invention also relates to the ultrahigh molecular weightpolyethylene obtainable by the process according to the invention.

The particulate UHMWPE that may be obtained by the process of thepresent invention has good process ability while being polymerized witha good productivity of the employed supported organometallic compound.Hereby, for the first time according to the knowledge of the inventors,a particulate UHMWPE has been provided with an optimized combination ofprocess ability and UHMWPE characteristics such as molecular weightdistribution, particles size and catalyst residues. Optionally particlesize distribution and particle morphology may be optimized.

Hence the present application also relates to a particulate ultra highmolecular weight polyethylene (pUHMWPE) having an intrinsic viscosity(IV) of at least 4 dl/g, a molecular weight distribution Mw/Mn of lessthan 4, a median particle size D50 of between 50 and 200 μm, a residualTi-content of less than 10 ppm, a residual Si-content of less than 50ppm, and a total ash content of less than 1000 ppm. Preferably theSi-content of the pUHMWPE according to the invention is less than 40,more preferably less than 30, even more preferably less than 25, evenmore preferably less than 20 and most preferably less than 10 ppm.

The ultra-high molar weight polyethylene according to the invention hasan intrinsic viscosity (IV, as measured on solution in decalin at 135°C.) of at least about 4 dl/g, preferably at least about 8, morepreferably at least about 12 dl/g, to provide articles comprising thepUHMWPE with optimal mechanical properties. The pUHMWPE according to theinvention may have an IV of at most 50 dl/g, preferably at most 40 dl/g.Intrinsic viscosity is a measure for molecular weight (also called molarmass) that can more easily be determined than actual molecular weightparameters like M_(n) and M_(w). There are several empirical relationsbetween IV and Mw. Based on the equation M_(w)=5.37*10⁴ [IV]^(1.37) (seeEP 0504954 A1) an IV of 4 or 8 dl/g would be equivalent to M_(w) ofabout 360 or 930 kg/mol, respectively. When the intrinsic viscosity istoo small, the strength necessary for using various molded articles fromthe ultrahigh molecular weight polyethylene sometimes cannot beobtained, and when it is too large, the process ability, etc. uponmolding is sometimes worsen.

The pUHMWPE according to the invention has a molecular weightdistribution Mw/Mn of less than 4.0, preferably less than 3.5,preferably less than 3 and even most preferably less than 2.8. Suchpreferred pUHMWPE may demonstrate optimized mechanical properties. Bymolecular weight distribution (MWD) in the context of the presentapplication is understood the ratio of Mw/Mn. Since there may beconflicting teachings in the literature about the way to measure M_(w)and/or M_(n) values for a particular UHMWPE, resulting in a discrepancyof the MWD, the herein understood MWD is the one as measured by SECtechnique as further described in the experimental section. The MWD ofthe particulate UHMWPE according to the invention has no particularlylow limit. The theoretical limit is the one of a monodisperse polymerwith an MWD of 1, preferably the MWD is at least 1.1.

Preferably, the UHMWPE is a linear polyethylene with less than one longchain branch (LCB) per 100 total carbon atoms, and preferably less thanone LCB per 300 carbon atoms; an LCB is herein defined as a branchcontaining at least 20 carbon atoms.

The particulate UHMWPE of the invention has a median particle diameter(D50) from 50 to 200 micron. It is preferably from 60 to 180 micron,more preferably from 65 to 150 micron. Too low particle sizes lead todust and safety issues in the process whereas too high particle sizesmay negatively impact the process ability of the pUHMWPE by for exampleby uneven sintering.

In a further preferred embodiment, the particulate UHMWPE of theinvention has a particle size distribution parameter (SPAN) of at most3, preferably at most 2.5 and even more preferably at most 2 and mostpreferably at most 1. The SPAN is expressed by the equationSPAN=(D90−D10)/D50 wherein D90, D10 and D50 are particle sizes at 90%,10% and 50% in a volume cumulative distribution, respectively, while D50is also referred to as the median diameter. The smaller the value is,the narrower the particle size distribution is.

The morphology of the particulate UHMWPE of the invention may beinfluenced by the magnesium containing carrier prepared during theprocess of the invention. It was observed that the supported catalystmay yield pUHMWPE of substantially spheroidal shape. By substantiallyspheroidal shape in the context of the present invention is meant thatthe largest diameter of an individual UHMWPE particle is at most 1.5times the average diameter of said particle, preferably at most 1.4times the average diameter and most preferably at most 1.2 times theaverage diameter of said particle. By largest diameter of a particle isunderstood the diameter of the smallest possible sphere that wouldcircumscribe the entire particle. By average diameter of a particle isunderstood the diameter of the sphere that can comprise the mass of theconcerned particle. Such spheroidal morphology stands for example incontrast to oblong or granular particles or particles of irregular shapeas often obtained from most of oxide-based carriers such as silicacarriers. Spheroidal morphology of the pUHMWPE particles providesproducts with good powder bulk density.

The particulate UHMWPE of the invention is further characterized by aresidual Ti-content of less than 10 ppm, preferably less than 7 ppm,more preferably less than 5 ppm and most preferably less than 2 ppm.Lower residual Ti levels are a result of an increased productivity ofthe supported catalyst and may allow pUHMWPE with an increasedstability.

In one preferred embodiment, the level of any residual active catalystmetal is less than 10 ppm, preferably less than 8 ppm, more preferablyless than 7 ppm and most preferably less than 5 ppm. In a yet preferredembodiment, the combined level of residual active catalyst metals isless than 25 ppm, preferably less than 20 ppm, more preferably less than15 ppm and most preferably less than 10 ppm. By active catalyst metal isherein understood metals that are commonly used for the production ofpolyolefins, especially the metals of groups 4 to 7 of the periodictable of elements, more especially the metals of group 4, i.e. Ti, Zrand Hf. The respective levels of Ti, Zr, and/or Hf are measured byNeutron Activation Analysis (NAA).

The particulate UHMWPE of the invention is further characterized by aresidual ash content of less than 1000 ppm, preferably less than 600ppm, more preferably less than 300 ppm and most preferably less than 100ppm. Lower residual ash levels are a further result of an increasedproductivity of the supported catalyst and may provide pUHMWPE havingimproved color neutrality. The residual ash content of the particulateUHMWPE is calculated based on the amount of supported catalyst in mg andthe respective polyethylene yield of said catalyst in kg during thepolymerization. The skilled person will be aware that residual ashcontent can also be approximated by the sum of the elements identifiedby Neutron Activation Analysis (NAA) or X-ray fluorescence (XRF) orother analytical means available in the art.

The particulate UHMWPE may be prepared by a catalyst supported by amagnesium containing carrier as described above. Such carrier as well astherefrom prepared UHMWPE is substantially free of silica, though smallamounts of silicon may be present in the pUHMWPE depending upon the usedchlorinating agent. Hence the pUHMWPE of the present invention has aresidual Si-content of less than 50 ppm, preferably less than 40 ppm,more preferably less than 30 ppm and most preferably of less than 20ppm. The residual Si-content of the particulate UHMWPE is calculatedbased on the amount of Silicium (in mg) present in the supportedcatalyst and the respective polyethylene yield of said catalyst in kgduring the polymerization. As above, NAA and XRF are suitable method toconfirm residual Si levels.

In a yet further preferred embodiment, the pUHMWPE of the invention hasan apparent bulk density of at least 300 kg/m³. It is preferably from350 to 550 kg/m³, more preferably from 380 to 530 kg/m³, further morepreferably from 390 to 520 kg/m³. It was found that pUHMWPE with bulkdensities in the mentioned ranges may provide good handlingcharacteristics.

The particulate ultrahigh molecular weight polyethylene of the presentinvention is eminently suitable for the manufacture of molded articlessuch as tapes, films and fibers. Accordingly, the present invention alsopertains to a process for manufacturing of molded UHMWPE articles,preferably tapes, films and fibers from the pUHMWPE of the invention.

A suitable method for producing a molded article from the pUHMWPEaccording to the invention is a gel spinning process as described in forexample GB-A-2042414, GB-A-2051667, EP 0205960 A and WO 01/73173 A1, andin “Advanced Fibre Spinning Technology”, Ed. T. Nakajima, Woodhead Publ.Ltd (1994), ISBN 185573 182 7. In short, the gel spinning processcomprises preparing a solution of a polymer of high intrinsic viscosity,extruding the solution into a tape, film of fiber at a temperature abovethe dissolving temperature, cooling down the tape, film of fiber belowthe gelling temperature, thereby at least partly gelling the tape, filmor fiber, and drawing the film, tape or fiber before, during and/orafter at least partial removal of the solvent.

The present application also pertains to products which contain a moldedarticle of the invention, such as ropes, cables, nets, fabrics, andprotective appliances such as ballistic resistant articles.

The present invention will be elucidated by the following examples,without being limited thereto or thereby.

EXPERIMENTAL

The organometallic compound Cp*Ti[(t-Bu)₃PN]Cl₂ (I) was producedaccording to the method described in Douglas W. Stephan et al inOrganometallics, 2003, 22, 1937-1947, which is hereby included byreference.

Preparation of Magnesium Containing Carrier and Supported CatalystExample 1: Preparation of the Magnesium Containing Carrier

In a 1 L reactor equipped a thermostat, internal temperature control anda mechanical stirrer. 130 mL of PhMgCl in dibutylether (0.53 mol Mg/L)was stirred at 500 rpm at 10° C. 75 mL of a 3.7 M solution of PhSiCl₃ inPhCl was added drop wise at a rate of 75 ml/hour. The reaction mixturewas stirred for 30 minutes at 10° C., then heated at a rate of 1° C./min to 60° C., and finally stirred for a further 30 minutes at 60° C.Heptane washings were then performed until the supernatant is clean. Thesupport obtained has a median particle size 10.2 μm of and a span of0.96 as measured by Malvern Laser light scattering.

Example 2: Pre-Treatment of the Magnesium Containing Carrier by an AlkylAluminum Compound

A toluene suspension of the MgCl₂ support from Example 1 andMAO_(10% wt) (0.138 mol in toluene) was stirred at 300 rpm and 60° C.for 1 hour. The resulting solid was washed thoroughly with toluene at60° C. until the supernatant is clean.

Example 3: Supporting of the Organometallic Compound

In a 1 L reactor, ˜5 g of solid from Example 2 in toluene is stirred at300 rpm and at room temperature. 40 mL of 0.02 M toluene solution ofcompound (I) is added and the mixture is allowed to react for 1 hour.Toluene washings are then performed until the supernatant is colorless.The solid is finally slurried in 250 mL of heptane.

Example 4

In a 100 mL reactor, ˜250 mg of solid from Example 2 in toluene iscontacted at room temperature with 20 mL of 0.015 M toluene solution ofcompound (I) for 1 hour. Toluene washings are then performed until thesupernatant is colorless. The solid is finally slurried in 2 mL ofheptane.

Example 5

Supported catalyst is prepared as described in example 3 from a supportwith a d50 of 5.5 μm obtained by the process of example 1 at a higherstirring rate.

Example 6

Supported catalyst is prepared as described in example 3 from a supportwith a d50 of 3.2 μm.

Example 7: Preparation of the Magnesium Containing Carrier

The process of Example 1 is repeated with the difference that the PhMgClwas at a molar rate of 1.0 mol Mg/L. The support obtained has a medianparticle size 6.5 μm of and a span of 0.92 as measured by Malvern Laserlight scattering.

Example 8

A supported catalyst is prepared as described in Example 3 from asupport prepared as described in Example 9.

Example 9

A supported catalyst is prepared as described in Example 3 from asupport with a d50 of 11.9 μm obtained by the process of Example 9whereby the stirring rate is 250 rpm.

Example 10

Supported catalyst is prepared as described in Example 3 from a supportwith a d50<2 μm obtained by the process of Example 9 whereby thestirring rate is 1400 rpm.

Comparative Experiment A

Catalyst prepared as described in example 4 except that the solution ofcompound (I) is replace by 20 mL of a 0.0125 M toluene solution ofCpTiCl₃.

Comparative Experiment B

Catalyst prepared as described in example 4 except that the solution ofcompound (I) is replace by 20 mL of a 0.015 M toluene solution ofMe₅CpTiCl₂(NC(2,6-F₂Ph)(Pr₂N) prepared as described in WO2005/090418.

Comparative Experiment C

A MgCl₂ support is prepared as described in example 18 of U.S. Pat. No.7,528,091B2. The resulting d50 was 12.5 μm. 250 mg of the resultingsolid in toluene is contacted at room temperature with 20 mL of 0.015 Mtoluene solution of compound (I) for 1 hour. Toluene washings are thenperformed until the supernatant is colorless. The solid is finallyslurried in heptane.

Comparative Experiment D

A MgCl₂ support prepared as described in Example 1 slurried in heptaneis contacted at 60° C. with 0.09 mol TiCl₄ for 1 h. Heptane washingswere then performed until the supernatant is clean.

General Polymerization Procedure:

Unless otherwise specified, batch polymerizations were carried out in astirred 2 L or 10 L reactor. The reaction temperature was set to therequired temperature and controlled by a Lauda thermostat. The feedstreams (solvent and ethylene) were purified with various adsorptionmedia to remove catalyst killing impurities such as water, oxygen andpolar compounds as is known by someone skilled in the art. In an inertatmosphere the previously dried reactor is filled with 1 L of heptane(or 4.5 L for Experiments IV and VI). After the solvent has reached thedesired temperature, the scavenger components are added and after 5minutes the supported catalyst is added. Next the ethylene stream is fedinto the reactor to reach and maintain a total pressure of 0.5 or 1.0MPa. After the desired polymerization time, the contents of the reactoris collected, filtered and dried under vacuum at 50° C. for at least 12hours. The polymer is weighted and samples are analyzed.

Scavenger Productivity Polymerization TEA Temperature Pressure ReactionYield Cat Yield [gpol/gcat * Reaction Catalyst Cat [mg] [mmol/L] [° C.][MPa] Time [h:m] [g] [gpol/gcat] h * barg] D50 [μm] A C. Exp. A 250 1.0060 0.5 2:04 196 787 76 105.9 B C. Exp. B 250 1.00 60 0.5 2:00 35 140 14C C. Exp. C 250 1.00 60 0.5 2:00 55 220 22 618.0 D C. Exp. D 15 0.92 600.5 2:00 98 6528 653 171.9 I Ex. 3 100 1.00 60 0.5 2:00 259 2589 259165.4 II Ex. 4 250 1.00 60 0.5 1:34 358 1437 184 129.2 III Ex. 5 1000.92 60 0.5 2:00 301 3009 301 89.2 IV Ex. 5 110 4.6 70 1.0 3:53 172015686 145.1 V Ex. 6 15 0.92 60 0.5 1:00 266 17686 3537 57.6 VI Ex. 6 504.6 60 0.5 7:00 1325 26225 93.5 VII Ex. 8 15 0.92 60 0.5 2:00 147 9737974 101.6 VIII Ex. 9 15 0.92 60 0.5 2:00 79 5204 520 166.4 IX Ex. 10 150.92 60 0.5 2:00 340 22603 2260 41.5 Bulk Calculated Polymerizationdensity Ti content Ash Content DF ES IV Mn Mw Mw/Mn Reaction SPAN [g/L][ppm] [ppm] [s] [N/mm²] [dL/g] [kg/mol] [kg/mol] [—] A 0.74 284 1271 450.379 360 3000 8.2 B 193 7143 380 2400 6.3 C 1.94 4545 430 2400 5.7 D1.12 321 153 30 0.423 470 3000 6.3 I 0.8 340 386 28 20.8 960 3100 3.2 II0.73 346 4.5 696 31 0.433 1100 2800 2.5 III 0.82 360 332 35 — — — — IV0.83 456 64 25 0.488 — — — — V 0.98 298 57 45 1100 3400 2.9 VI 0.92 41838 32 3800 1500 2.5 VII 0.90 370 <1.7 103 38 VIII 0.82 362 1.9 192 22 IX1.04 320 0.4 44 0.469Test MethodsSEC-MALS:

The molecular mass distributions (Mn, Mw, Mz, Mw/Mn) were measured usinga PL-210 Size Exclusion Chromatograph coupled to a refractive indexdetector (PL) and a multi-angle light scattering (MALS) detector (laserwavelength 690 nm) from Wyatt (type DAWN EOS). Two PL-Mixed A columnswere used. 1,2,4-trichlorobenzene was used as the solvent, the flow ratewas 0.5 ml/min, and the measuring temperature was 160° C. Dataacquisition and calculations were carried out via Wyatt (Astra)software. The UHMWPE should be completely dissolved under suchconditions that polymer degradation is prevented by methods known to aperson skilled in the art.

Bulk density is determined according to DIN 53466; ISO 60 at 23° C. and50% relative humidity.

Particle Size and Span:

The average particle size (d50) of the polymer is determined inaccordance with ISO 13320-2, using a Malvern™ LLD particle sizeanalyzer. The span defined as (d90-d10)/d50 was also determined usingthe Malvern™ LLD particle size analyzer.

The average size of the catalyst is determined using a Malvern™ LLDparticle size analyzer.

Dry Flow (DF):

The dry flow in seconds was measured according to the method describedin ASTM D 1895-69, Method A; 23° C. and 50% relative humidity.

Intrinsic Viscosity (IV):

The Intrinsic Viscosity is determined according to method PTC-179(Hercules Inc. Rev. Apr. 29, 1982) at 135° C. in decalin, thedissolution time being 16 hours, with BHT (Butylated Hydroxy Toluene) asanti-oxidant in an amount of 2 g/l solution, by extrapolating theviscosity as measured at different concentrations to zero concentration;

Elongational Stress (ES)

of UHMWPE is measured according to ISO 11542-2A.

The invention claimed is:
 1. A particulate ultra high molecular weightpolyethylene (pUHMWPE) having: an apparent bulk density of at least 300kg/m³, an intrinsic viscosity (IV) of at least 4 dl/g to at most 50dl/g, a molecular weight distribution Mw/Mn of less than 4.0, a medianparticle size D50 of between 50 and 200 μm, a residual Ti-content ofless than 10 ppm, a residual Si-content of less than 50 ppm, and a totalash content of less than 1000 ppm.
 2. The pUHMWPE according to claim 1,wherein the polymer has a Mw/Mn of less than 3.5.
 3. The pUHMWPEaccording to claim 1, wherein the polymer has a Mw/Mn of less than
 3. 4.The pUHMWPE according to claim 1, wherein the polymer has a Mw/Mn ofless than 2.8.
 5. The pUHMWPE according to claim 1, wherein the polymerhas an IV of at least about 8 dl/g to at most 50 dl/g.
 6. The pUHMWPEaccording to claim 1, wherein the polymer has an IV of at least about 12dl/g to at most 40 dl/g.
 7. The pUHMWPE according to claim 1, whereinthe pUHMWPE has a SPAN of at most
 3. 8. A process for manufacturing amolded UHMWPE article comprising the step of molding the pUHMWPE ofclaim 1 to obtain a molded article therefrom.
 9. The process accordingto claim 8, wherein the molded article is a fiber, tape or film.
 10. Amolded article comprising the pUHMWPE according to claim 1, wherein themolded article is selected from the group consisting of ropes, cables,nets, fabrics, and protective appliances.
 11. The molded articleaccording to claim 10, wherein the molded article is a ballisticresistant article.